1. Field of the Invention
This invention relates to an image processing technology using binary image data and contone image data both of which are based on the same image to perform a color correction or a density correction with respect to the binary image data for providing binary image data after subjected to the correction.
2. Description of the Related Art
In recent years, a method called CTP (computer to plate) using digitalized print original data to directly make a machine plate for printing by a digital plate making apparatus has been adopted in a plate making process. To make a machine plate for printing according to the CTP, RIP (Raster Image Processing) is performed for the present data described in a page description language to prepare contone (color) bit map data, and the bit map data is converted into binary image data having a dot component for each color component by performing screen processing and then a machine plate is made from the binary image data. If a machine plate is made by the CTP, when the present proofreading is performed, the image data binarized by the RIP usually is output to a color proofreading system called DDCP (direct digital color proof), whereby a galley proof is provided.
In recent years, remote color proofreading for easily providing a galley proof in each company, etc., has been performed without using an expensive machine such as the DDCP. In the remote color proofreading, a galley proof is provided using a printer installed in each company, etc. However, since the printers differ in characteristics, color conversion is performed in each printer for providing a galley proof so that output is produced in the same colors regardless of the printer. Accordingly, press run and the galley proof are almost matched with each other in color appearance and color proofreading work is accomplished reliably. Under present circumstances, color adjustment is made in the state of the bit map data and output is produced according to the output system of each printer, whereby a galley proof is provided.
However, even if color match is accomplished, when print is executed, the proof galley and the press run may differ in appearance depending on the screen put at the plate making time. Thus, it is desirable that a galley proof subjected to screen processing similar to that in press run should be provided. To do this, binary image data after subjected to screen processing may be received and the color reproduction difference between the printers may be corrected before a galley proof is produced. Such attempts have been made.
FIG. 12 is a block diagram to show an example of an image processing apparatus according to a related art. In the figure, numeral 81 denotes RIP means, numeral 82 denotes contone conversion means, numeral 83 denotes descreening means, numeral 84 denotes a color correction table, numeral 85 denotes color correction means, numeral 86 denotes error diffusion processing means, and numeral 87 denotes output means.
First, the RIP means 81 converts data into bit map data and performs screen processing for the bit map data to provide binarized image data D, and supplies the image data D to the contone conversion means 82. The image data D has a dot component as the screen processing is performed. Next, the contone conversion means 82 artificially converts the supplied binary image data D into contone image data Do, and the descreening means 83 performs descreening processing for the image data Do, whereby the dot component is removed from the image data Do to generate image data Dd having halftone.
Next, color correction data Dc to make a color correction to the image data Dd is read from the color correction table 84. The color correction means 85 adds the color correction data Dc and the contone image data Do having a dot component and image data Dx provided by adding the data is input to the error diffusion processing means 86, which then performs error diffusion processing for the input image data, thereby converting the contone image data to binary image data Dz, and outputs the provided image data to the output means 87. The image data thus output has the original dot component and becomes image data subjected to color matching, so that a highly accurate galley proof can be provided.
In the system, however, as the correction value is added, ON and OFF pixels produced by the error diffusion processing are produced independently of the original halftone-dot structure. Therefore, the ON and OFF pixels produced independently of the original halftone-dot structure are conspicuous when the correction value is large or in a low-density area or a high-density area; this is a problem. A white patch occurs in a dot and is conspicuous and thus the image quality of the whole image is degraded. Since the error diffusion processing means 86 assumes the range of pixel values of a usual contone image, for example, if a negative value as the color correction data Dc is added to 0% pixel of the contone image data Do, correction to 0% is made and the color processing correction result cannot precisely be reflected. In fact, the contone image data Do contains only density 0% and 100% pixels and the color correction data Dc has positive and negative values and thus the pixel value range is often exceeded by adding the correction value to a simple contone image, in which case no correction is made; this is a problem.
It is known to provide a technology wherein when a binary image containing a halftone-dot structure is converted into a contone image, an appropriate filter coefficient is used, whereby the binary image is converted into a contone image so as to retain the halftone-dot structure, and error diffusion processing is performed for the contone image with the halftone-dot structure retained for again converting the contone image into a binary image and a galley proof is produced.
In the system, a diffusing filter called soft focus is used and a weak low-pass filter to such an extent that the dot shape can be maintained is applied. However, because of the diffusing filter, the shape of a small dot as in a low-density or high-density area is not left and an almost uniform contone image results and if the contone image is again converted into a binary image, a white patch or a crush occurs. As an ON or OFF pixel appears in each of pixels making up a grid of dots also in an intermediate-density area, the cluster of the pixels making up the grid of dots is crushed. Thus, the image quality is degraded and the difference between the galley proof and the press run grows. Further, to convert the binary image into a contone image while retaining various halftone-dot structures, the binary image must be analyzed to set an appropriate filter coefficient; this is also a problem.
Further, it is known to provide a technology of performing color correction processing fitted to the characteristics of a color printer for binary image data. In this art, first, after binary image data is converted into contone image data, color correction processing is performed so as to fit to the characteristics of the color printer and then the contone image data subjected to the color correction processing is binarized and the number of print pixels fitted to the contone data is determined. When the number of print pixels is determined, the pixel placement of the binary image data is referenced, whereby the print pixels are placed. If the number of print pixels is increased or decreased, a table is referenced and pixels at what positions are to be increased or decreased is determined. An error occurring when pixels are placed, particularly an error occurring in conversion from contone data to binary data is diffused to other pixels.
In the image data processing method, as the processing is performed, the binary image data most suitable for the color printer is generated and an image close to the present image can be provided. However, since a predefined table is used when the number of dots of the binary image is increased or decreased, if the periods differ between tables in determining the priority of increasing or decreasing the number of dots and the dots of the binary image, interference occurs; this is a problem.